Transformer Calculator

What a transformer does

A transformer moves power from one voltage to another using nothing but magnetism. Two coils of wire share an iron core. Alternating current in the first coil builds and collapses a magnetic field, that moving field pushes current in the second coil, and the ratio of turns between the two coils sets the voltage on each side. No moving parts, no electronics, and no change in frequency: a 60 Hz input stays 60 Hz on the way out.

Power in is close to power out. When voltage drops, current rises, so a transformer that cuts 480 volts down to 120 volts multiplies the available current by 4 at the same time. That trade is the whole point. Utilities push power across long lines at tens of thousands of volts, where current is small and losses are low, then step it down in stages until a pole unit or a dry-type box delivers the 240 or 120 volts a house needs.

Every transformer carries a kVA rating instead of a watt rating, because the manufacturer cannot know the power factor of whatever you plug into it. The rating says how many volt-amps the coils can carry without overheating. This calculator works in kVA, volts, and amps, so the numbers line up with what is printed on the nameplate.

Give it a rating and the two voltages and it returns the full-load current on both sides plus the turns ratio. Add an efficiency and it estimates the watts that turn into heat. Everything runs on the page, nothing is uploaded.

The formulas: turns ratio, volts, and amps

Four short formulas cover the whole page. The turns ratio follows the voltages, the currents follow the kVA rating, and the two are linked so that volts times amps stays the same on both sides, less a small loss.

Turns ratio Np / Ns = Vp / Vs, the primary turns divided by the secondary turns, which equals the primary voltage divided by the secondary voltage. Full-load current on one phase is I = kVA × 1000 ÷ V. On three-phase systems the line current is I = kVA × 1000 ÷ (1.732 × V), where 1.732 is the square root of 3. Currents scale inversely: Ip / Is = Vs / Vp, so a 4 to 1 step-down makes the secondary current 4 times the primary current.

The U.S. Department of Energy test procedure and the efficiency rules behind these units live in federal regulation, and the DOE distribution transformer program page is the plain-language entry point for them.

Worked example: a 25 kVA, 480 to 120 volt unit

Take a common shop transformer rated 25 kVA, single-phase, with 480 volts on the primary and 120 volts on the secondary. The primary full-load current is 25,000 ÷ 480 = 52.08 amps. The secondary full-load current is 25,000 ÷ 120 = 208.33 amps. The turns ratio is 480 ÷ 120 = 4, written 4 to 1, and sure enough 208.33 is 4 times 52.08.

That secondary current is why wire size matters. A 25 kVA unit pushing 208 amps needs a heavy conductor and a properly sized breaker on the low-voltage side, while the primary side carries a fifth of that current. Anyone who wires the secondary with primary-sized wire learns this the hard way.

The same math in a smaller package: a 5 kVA control transformer stepping 240 volts to 120 volts draws 5,000 ÷ 240 = 20.83 amps on the primary and delivers 5,000 ÷ 120 = 41.67 amps on the secondary, a 2 to 1 ratio. Step up instead of down and the roles swap, with the high-voltage coil carrying the smaller current.

Three-phase current and the 1.732 factor

Three-phase power adds a factor. In a three-phase system the three coil voltages peak one after another, and the line current ends up equal to kVA × 1000 ÷ (1.732 × V), where V is the line-to-line voltage. The 1.732 is the square root of 3, and forgetting it is the most common error people make with three-phase ratings.

A 75 kVA three-phase unit on a 480 volt service carries 75,000 ÷ (1.732 × 480) = 90.21 amps at full load. On the secondary, feeding a 208 volt panel, the same 75 kVA delivers 75,000 ÷ (1.732 × 208) = 208.19 amps. Drop the 1.732 and both answers come out 73 percent high, big enough to order the wrong breaker and the wrong wire.

The 1.732 shows up on the nameplate too. A panel marked 208Y/120 or 480Y/277 is a three-phase wye system, and the higher number is the line-to-line voltage the formula wants. Plug the line-to-line value into the calculator, set the phase switch to three-phase, and the current on the screen is the line current an electrician measures with a clamp meter.

Efficiency and where the losses go

Nothing is free. Losses in a transformer come from two places: copper losses in the wire, which grow with the load, and core losses, which run whenever the unit is energized whether or not anything is connected. At 98.60 percent efficiency a 75 kVA unit wastes 75,000 × (100 ÷ 98.60 - 1) = 1,065 watts at full output, and that heat has to leave through the case or the oil.

Federal rules set a floor on those losses. Since January 1, 2016, distribution transformers sold in the United States must meet the minimum efficiency levels in 10 CFR 431.196, and the regulation text on govinfo.gov lists every rating. Bigger units do better than small ones, which is why the required efficiency climbs with kVA.

One detail trips people up: the DOE rating is measured at 50 percent of nameplate load, not at full load, because that is where the copper and core losses balance out. Run the same 75 kVA unit at 37.5 kVA and the same 98.60 percent figure implies 37,500 × (100 ÷ 98.60 - 1) = 532 watts of loss. The calculator takes whatever efficiency you type and reports the losses at the kVA you type, so keep the pair consistent.

Reading the nameplate

The nameplate answers most questions before they start. The kVA rating, the primary and secondary voltage, and the phase are the three you need for this calculator. Below those you will usually find taps, marked like 480 - 456 - 432 or plus and minus 2.5 percent steps, which let you trim the output a few percent when the supply voltage runs high or low.

Impedance appears as a percent, often somewhere between 2 and 6 percent on distribution sizes. It tells you how much the secondary voltage sags under load and how much fault current the unit can feed, which sets the ratings of the breakers downstream. Temperature rise, printed as a number like 150 °C, says how much hotter the windings run at full load, and it pairs with an insulation class.

Frequency is on the plate too. A 60 Hz unit works on a 60 Hz supply, and a transformer built for 50 Hz will run hot on nothing else without a derate, so import equipment deserves a second look. Enclosure type matters in the field: dry-type units live indoors in ventilated boxes, while oil-filled and pad-mounted units sit outside, and the cooling method, AN, AA, OA, or ONAN, is printed in the same block.

None of these markings change the arithmetic on this page, but they explain why two transformers with the same kVA can look nothing alike once cost, cooling, and mounting are factored in.

Sizing rules and safety

Sizing starts with the load. Add up the volt-amps of everything the transformer will feed, not just the watts, then leave headroom. A common habit keeps a continuous load near 80 percent of the nameplate rating so the unit runs cool and lasts longer; motor loads need extra margin because starting current is several times running current. The National Electrical Code, in its transformer article 450, sets the overcurrent protection rules that follow from the impedance and the rating.

On the wiring side, the current this calculator reports is the number the conductor and the breaker must handle, and the two sides rarely match. A transformer that steps voltage down raises the current by the same ratio, so the low-voltage side is almost always the one that needs the bigger copper.

Safety is not optional. Kill and lock out the supply before touching a terminal, confirm with a meter, and bond the secondary per the code that governs the job. Transformers hold stored energy and can feed serious fault current even when the load is small, and a licensed electrician is the right person to make the final call on protection and grounding.

Use the calculator to check the arithmetic before you buy or wire anything: full-load current on each side, the turns ratio, and the heat the unit will shed at the efficiency printed on its plate.